1. Field of the Invention
The present invention relates to a Nb3Sn superconducting wire manufactured by an internal Sn process, and to a precursor for manufacturing such a Nb3Sn superconducting wire. Specifically, the present invention relates to a technique for manufacturing a Nb3Sn superconducting wire useful as a material of a superconducting magnet for generating a high magnetic field.
2. Description of the Related Art
In the field in which superconducting wires are put into practical use, with respect to superconducting magnets used for high-resolution nuclear magnetic resonance (NMR) spectrometers, nuclear fusion devices, and accelerators, the resolution increases as the generated magnetic field increases. Thus, there has recently been the tendency that magnetic fields generated by superconducting magnets increase more and more.
As a superconducting wire used for superconducting magnets generating a high magnetic field, a Nb3Sn wire has been put into practical use, and a bronze process is mainly utilized for manufacturing such a Nb3Sn superconducting wire.
In the bronze process, a plurality of core materials composed of Nb or a Nb-based alloy is embedded in a Cu—Sn-based alloy (bronze) matrix to form a composite wire. The composite wire undergoes a reduction process such as extrusion or wire drawing, so that the core materials are thinned down to form filaments (hereinafter referred to as “Nb-based filaments”). Next, a plurality of the composite wires each including the Nb-based filaments and the bronze matrix are bundled to form a wire group and then coated with copper for stabilization (stabilizing copper), after which the wire group further undergoes the reduction process. After the reduction process, the above wire group undergoes a heat treatment (diffusion heat treatment) at 600° C. to 800° C., so that a Nb3Sn compound layer is produced at a boundary between the Nb-based filaments and the bronze matrix.
However, the bronze process is disadvantageous in that the solid solubility of Sn in bronze has a limit (15.8% by mass or less), so that the Nb3Sn compound layer has a relatively small thickness, and crystallinity deteriorates to degrade high magnetic properties.
As a method of manufacturing a Nb3Sn superconducting wire other than the bronze process, an internal Sn process is known. Unlike the bronze process, the internal Sn process (also called as an “internal diffusion process”) has no limit in terms of Sn concentration due to a solid solubility limit. Therefore, the Sn concentration can be set as high a value as possible, which enables the production of a Nb3Sn layer of high quality, thereby obtaining a high critical current density Jc at a high magnetic field.
In the internal diffusion process, as shown in FIG. 1 (a schematic view of a precursor for manufacturing a Nb3Sn superconducting wire), a core composed of Sn or a Sn-based alloy (hereinafter may be referred to as a “Sn-based metal core”) 3 is embedded in the center of a Cu or Cu-based alloy (hereinafter may be referred to as a “Cu matrix”) 4. In the Cu matrix 4 surrounding the Sn-based metal core 3, a plurality of core materials composed of Nb or a Nb-based alloy (i.e., “Nb-based filaments”) 2 is arranged so as to be spaced from each other, thereby preparing a precursor (a precursor for manufacturing a superconducting wire) 1. The precursor undergoes a wire drawing process and then a heat treatment (diffusion heat treatment), so that Sn in the Sn-based metal core 3 is diffused and reacts with the Nb-based filaments 2, thereby producing Nb3Sn (see Japanese Unexamined Patent Application Publication No. 49-114389).
As shown in FIG. 2, such a precursor 1 generally includes a portion (hereinafter may be referred to as a “superconducting matrix portion”) where the Nb-based filaments 2 and the Sn-based metal core 3 are disposed, a stabilizing copper layer 4a provided outside the superconducting matrix portion, and a diffusion barrier layer 6. The diffusion barrier layer 6 is disposed between the matrix portion and the stabilizing copper layer 4a. The diffusion barrier layer 6 is, for example, a Nb layer, a Ta layer, or a double layer including a Nb layer and a Ta layer, and serves to prevent the Sn (Sn-based metal core 3) in the superconducting matrix from diffusing outside during the diffusion heat treatment, thereby enhancing the purity of Sn in the superconducting matrix.
The above-described precursor for manufacturing a superconducting wire is manufactured by following steps: First, Nb-based filaments are inserted into a Cu matrix tube, and the reduction process such as extrusion or wire drawing is performed to form a complex (generally having a hexagonal section) which is then cut into a proper length. Next, a plurality of the complexes is filled into a billet which includes a Cu outer cylinder and is provided with or without the diffusion barrier layer 6. Subsequently, a Cu matrix (Cu solid billet) is disposed in the center of the billet and extrusion is performed. Finally, the Cu matrix in the center is mechanically perforated to prepare a pipe-like complex. Alternatively, a hollow billet which includes a Cu outer cylinder and a Cu inner cylinder and is provided with or without the diffusion barrier layer 6 is filled with a plurality of the above complexes (between the outer cylinder and the inner cylinder). Subsequently, hollow extrusion is performed to prepare a pipe-like complex.
In the gap formed in the center of the pipe-like complex prepared by the above-described process, a Sn-based metal core is inserted and subjected to the reduction process, thereby manufacturing a precursor shown in FIGS. 1 and 2. Hereinafter, the precursor may be referred to as a “mono-element wire”.
A plurality of the precursors (mono-element wire) prepared as described above is filled into a Cu matrix tube provided with or without the diffusion barrier layer 6, and then a reduction process is performed to form a precursor for manufacturing a multi-core superconducting wire (hereinafter, may be referred to as a “multi-element wire”).
FIGS. 3 and 4 illustrate examples of the multi-element wire. FIG. 3 illustrates a multi-element wire 11 in which a plurality of the precursors 1 (mono-element wire) shown in FIG. 1 are embedded in a Cu matrix 5 provided with a diffusion barrier layer 6a. FIG. 4 illustrates a multi-element wire 11a in which a plurality of the precursors (mono-element wire) shown in FIG. 2 are embedded in a Cu matrix 5 provided without a diffusion barrier layer.
In manufacturing a superconducting wire by the internal diffusion process using the above-described precursors, a technique disclosed in Japanese Patent No. 3273953 is known, for example, as a technique for improving properties (above-described critical current density Jc) of the resulting superconducting wire.
In this technique, for facilitating diffusion of Sn (diffusion from a Sn metal core) into Nb-based filaments, a wire has been proposed, in which in a final cross-sectional shape (sectional shape after the reduction process and before the diffusion heat treatment), the diameter of the Nb-bases filaments is within the range of about 1 to 3 μm, or the average cross-sectional area of the mono-element wire (specifically, superconducting matrix portion) is in the range of 0.0314 mm2 to 0.0019625 mm2. However, the superconducting wire manufactured using such a precursor has the problem of decreasing an n value as one of the superconducting properties.